|Publication number||US8160179 B2|
|Application number||US 12/028,402|
|Publication date||Apr 17, 2012|
|Filing date||Feb 8, 2008|
|Priority date||Feb 9, 2007|
|Also published as||US20080192859|
|Publication number||028402, 12028402, US 8160179 B2, US 8160179B2, US-B2-8160179, US8160179 B2, US8160179B2|
|Original Assignee||Texas Instruments Incorporated|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (10), Non-Patent Citations (5), Referenced by (1), Classifications (9), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims priority under 35 U.S.C. 119(a) to GB Provisional Application No. 0702626.3 filed Feb. 9, 2007.
This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/016,870 filed Dec. 27, 2007.
The present invention relates to cross-over conversion in data transfer applications.
High speed data transfer between semiconductor devices in an electronic system may be achieved by the use of a serialiser/deserialiser (SerDes). In order to avoid the use of a plurality of parallel connections between devices, a single differential analogue path is used running at a high data rate. One exemplary arrangement is specified by IEEE 802.3/AE/P.
In general, the differential signal from a transmitter arrives at an Analogue-Digital converter (ADC) in a receiver. In one exemplary application of the invention, the ADC has thirteen levels and output provides a 3-bit binary output, together with a sign bit.
High speed data transfer techniques such as SerDes operate in data transfer intensive environments, such as super computers and switch or router back planes. Signal routing in such environments is not trivial. Thus, it may not be possible to preserve the physical alignment of the tracks running between devices without recourse to track cross-over. A cross-over may be provided by a through board via or other wiring techniques, but not without a detrimental effect on the transmission channel characteristic and potential lack of symmetry. An alternative would be to wire directly from transmitter to receiver and to deal with the resultant inversion by means of a cross-over switch at either the transmitter output or the receiver input. However, the introduction of such a switch at the high data rates envisaged in the applications of the present invention would be problematic.
The present invention provides a receiver circuit comprising: a differential data input; a sign select input; an analogue-digital converter having inputs coupled to said differential data inputs and having first and second outputs, wherein said analogue-to-digital converter is adapted to convert a differential data signal received at said differential data input into a digital data output at said first output and a sign signal at said second output; and a selective sign inverter having a first input coupled to the sign output of said analogue-to-digital converter, a second input coupled to said sign select input and an output, wherein said selective sign inverter is adapted to provide a modified sign select signal at said output, wherein the signal received at the first input of said selective sign inverter is selectively inverted in dependence on the signal received at said second input in order to provide said modified sign select signal.
The present invention also provides a method comprising the steps of: receiving a differential data signal at a first input; receiving a sign select input; converting said differential data signal into a magnitude signal and a sign signal; selectively inverting said sign signal in dependence on the sign select input and providing said selectively inverted signal as a sign output; and outputting said magnitude signal and said sign output.
The selective sign inverter of the present invention may be an XOR gate, thereby providing a simple and elegant solution to the problem of data inversion caused by wiring cross-overs.
The digital data output may be in the form of a thermometer code. Further, a thermometer code to gray code converter may be provided for converting said digital data output into a gray code output.
In one form of the invention, the receiver is a SerDes receiver.
In one form of the present invention, the receiver described above forms parts of a transmitter-receiver pair comprising a transmitter having a differential data output and a sign select output and a receiver as described above, wherein the differential data output of said transmitter is coupled to the differential data input of said receiver and the sign select output of said transmitter is coupled to the sign select input of said receiver. The sign select signal provided by said transmitter is indicative of whether or not a wiring cross-over occurs between an output of said transmitter and an input of said receiver.
The present invention also provides a transmitter circuit comprising: a data input comprising a magnitude data input and a sign data input; a sign select input; a selective inverter having a first input coupled to the sign data input, a second input coupled to the sign select input and an output, wherein said selective sign inverter is adapted to provide a modified sign select signal at its output, wherein the signal received at the first input of said selective inverter is inverted in dependence on the signal received at the second input of said selective inverter in order to provide said modified sign select signal; a magnitude/sign to differential converter having a first input coupled to the magnitude data input, a second input coupled to an output of the selective inverter and an output, wherein said magnitude/sign to differential converter is adapted to convert the magnitude data input and the modified sign select data into a differential data signal and to provide said differential data signal at said output; and a differential output coupled to the output of said magnitude/sign to differential converter.
The said transmitter may comprise part of a transmitter-receiver pair. The transmitter can therefore be used with a receiver that does not have the selective inversion capability described above, thereby providing backward compatibility.
The present invention yet further provides a method comprising the steps of: receiving a data input comprising a magnitude data input and a sign data input; receiving a sign select input; selectively inverting said sign data input in dependence on the sign select input and providing said selectively inverted signal as a sign input to a magnitude/sign to differential converter; providing said magnitude data to said magnitude/sign to differential converter; and generating a differential output from said magnitude data and said selectively inverted signal.
Examples of the invention will now be described with reference to the accompanying drawings, of which:
A key challenge facing designers of high-bandwidth systems such as data-routers and super-computers is the requirement to transfer large amounts of data between ICs—either on the same circuit board or between boards. This data transmission application is called Serialisation-Deserialisation or “SerDes” for short. The present invention is useful in SerDes circuit and indeed was developed for that application. Nonetheless the invention may be used in other applications.
Analysis of typical backplane channel attenuation (which is around −24 dB) and package losses (−1 to −2 dB) in the presence of crosstalk predict that an un-equalized transceiver provides inadequate performance and that decision feedback equalization (DFE) is needed to achieve error rates of less than 10−17.
Traditional decision-feedback equalization (DFE) methods for SerDes receivers rely on either modifying, in analogue, the input signal based on the data history [“A 6.25 Gb/s Binary Adaptive DFE with First Post-Cursor tap Cancellation for Serial backplane Communications” R Payne et al ISSCC 2005; “A 6.4 Gb/s CMOS SerDes Core with feed-forward and Decision Feedback Equalization” M. Sorna et al ISSCC 2005; “A 4.8-6.4 Gb/s serial Link for Backplane Applications Using Decision Feedback Equalization” Balan et al IEEE JSSC November 2005.] or on having an adaptive analogue slicing level [“Techniques for High-Speed implementation of Non-linear cancellation” S. Kasturia IEEE Journal on selected areas in Communications. June 1991.] (i.e. the signal level at which the circuit decides whether the signal represents a 1 or a 0).
A block diagram of a SerDes receiver circuit 1, which forms part of an integrated circuit, in which the present invention may be used is shown in
In the receiver circuit 1 of
The receiver circuit 1 comprises two baud-rate sampling ADCs (analogue to digital converters) 2 and 3, a digital 2-tap FFE (feed forward equaliser) 4 and digital 5-tap DFE (decision feedback equaliser) 5 to correct channel impairments.
The SerDes section of the integrated circuit, which includes the receiver circuit 1 is also provided with a transmitter 40 (
The receiver 1 of
The digital samples output from the ADCs 2 and 3 are interleaved and the resulting stream of samples is fed into a custom digital signal processing (DSP) data-path that performs the numerical feed-forward equalization and decision-feedback equalization. This is shown in
The digital FFE/DFE is implemented using standard 65 nm library gates.
An advantage of applying the equalization digitally is that it is straightforward to include feed-forward equalization as a delay-and-add function without any noise-sensitive analogue delay elements. The FFE tap weight is selected before use to compensate for precursor inter-symbol interference (ISI) and can be bypassed to reduce latency. Whilst many standards require pre-cursor de-emphasis at the transmitter, inclusion at the receiver allows improved bit error rate (BER) performance with existing legacy transmitters.
The DFE 5 uses an unrolled non-linear cancellation method [“Techniques for High-Speed implementation of Non-linear cancellation” S. Kasturia IEEE Journal on selected areas in Communications. June 1991]. The data output (i.e. the 1s and 0s originally transmitted) is the result of a magnitude comparison between the output of the FFE 4 and a slicer-level dynamically selected from a set stored in a set 17 of pre-programmed registers. The values are determined by a control circuit (not shown in
The slicer-level is selected from one of 2n possible options depending on the previous n bits of data history. The history of the bits produced by the magnitude comparator 18 is recorded by a shift register 19 which is connected to shift them in. The parallel output of the shift register is connected to the select input of a multiplexer 20 whose data inputs are connected to the outputs of respective ones of the set 17 of registers holding the possible slicer-levels.
Unrolled tap adaption is performed using a least mean square (LMS) method where the optimum slicing level is defined to be the average of the two possible symbol amplitudes (+1−1) when proceeded by identical history bits. (For symmetry the symbols on the channel for the bit values 1 and 0 are given the values +1 and −1).
Although 5-taps of DFE were chosen for this implementation, this parameter is easily scaleable and performance can be traded-off against power consumption and die area. In addition, the digital equalizer is testable using standard ATPG (automatic test pattern generation) and circular built-in-self-test approaches.
The chosen clock recovery approach uses a Muller-Mueller approach [“Timing recovery in Digital Synchronous Data Receivers” Mueller and Muller IEEE Transactions on Communications May 1976.] where the timing function adapts the T/H sample position to the point where the calculated pre-cursor inter-symbol interference (ISI) or h(−1) is zero, an example being given in
A block diagram of the transmitter is shown in
A 4-tap FIR output waveform is obtained from simple current summing of the time-delayed contributions. This is done with differential amplifiers 45 to 48, each having its inputs connected to a respective one of the taps and having its differential output connected to a common differential output 49. Although shown as four differential amplifiers the circuit is implemented as one differential amplifier with four inputs, which minimizes return-loss. The relative amplitude of each contribution is weighted to allow the FIR coefficients to be optimized for a given circuit (e.g. a backplane) and minimize the overall residual ISI. The weights are determined empirically either for a typical example of a particular backplane or once a backplane is populated and are stored in registers 50 to 53. The weights respectively control the controllable driving current sources 54 to 57 of the differential amplifiers 45 to 48 to scale their output current accordingly. Respective pull-up resistors 58 and 59 are connected to the two terminals of the differential output 49.
A phase-locked loop (PLL) is used to generate low-jitter reference clocks for the transmitter and receiver to meet standardsflOIF-CEI-02.0—Common Electrical I/O (CEI)—Electrical and Jitter Interoperability agreements for 6G+ bps and 11G+ bps I/O″. Optical Internetworking Forum, February 2005; “IEEE Draft 802.3ap/Draft 3.0—Amendment: Electrical Ethernet Operation over Electrical Backplanes” IEEE July 2006.]. Most integrated circuits will have more than one receiver 1 and the PLL is shared between them with each receiver having a phase interpolator to set the phase to that of incoming data.
The PLL uses a ring oscillator to produce four clock-phases at a quarter of the line data-rate. The lower speed clocks allow power efficient clock distribution using CMOS logic levels, but need duty-cycle and quadrature correction at the point of use. The 3.125 GHz clocks are frequency doubled (XOR function) to provide the 6.25 GHz clock for the T/H & ADC. The transmitter uses the four separate 3.125 GHz phases, but they require accurate alignment to meet jitter specifications of 0.15UI p-p R.J. and 0.15U1 p-p D.J.
The system described has been fabricated using a 65 nm CMOS process and has been shown to provide error-free operation at 12.5 Gb/s over short channels (two 11 mm package traces, 30 cm low-loss PCB and two connectors). A legacy channel with −24 dB of attenuation at 3.75 GHz supports error free operation at 7.5 Gb/s.
A wiring cross-over of the form shown in
The circuit 800 comprises an ADC 802 and a selective inverter 804. The ADC receives a differential input signal across input terminals 806 and 808 and outputs a parallel data word to output terminal 810 and a sign bit to the input of selective inverter 804. The selective inverter 804 has a second input that receives a sign select input from input terminal 812.
The sign select input indicates whether or not a wiring cross-over has occurred. In the event that a wiring cross-over has occurred, the sign bit will be incorrect; in that event, the sign bit received by the selective inverter 804 is simply inverted under the control over the sign select input. If a wiring cross-over has not occurred, the sign bit will be correct and so the selective inverter 804 does not invert the sign bit.
In this manner, the circuit 800 compensates for the problem of sign inversion caused by wiring cross-over by performing a sign inversion at the receiver. In one embodiment of the invention, the selective inverter 804 is an XOR-gate.
In an alternative form of the invention, the selective inversion is carried out at the transmitter. A suitable transmitter 900 is shown in
The transmitter 900 comprises first 902, second 904 and third 906 input terminals, a selective inverter 908, a magnitude/sign to differential converter 910 and first 912 and second 914 output terminals. The first 902 and second 904 input terminals receive data for transmission by the transmitter 900. The first terminal 902 receives a multi-bit word indicating the magnitude of the data for transmission and the second terminal 904 receives a data bit indicating the sign of the data word for transmission. The third input terminal 906 receives a control signal indicating whether or not the connection between the transmitter 900 and the receiver with which it is to be used includes a wiring cross-over.
The second 904 and third 906 input terminals are connected to first and second inputs of the selective inverter 908. The selective inverter inverts the sign bit obtained from the second input terminal 904 if the signal on the third input terminal 906 indicates that a wiring cross-over is present. In one form of the invention, the selective inverter 908 is an XOR gate.
The magnitude/sign to differential converter 910 has a first input connected to the first input terminal 902 and a second input connected to the output of the selective inverter 908. The converter 910 converts the magnitude and sign data at its inputs into a differential output signal. The differential outputs of the converter 910 are connected to the first 912 and second 914 output terminals of the transmitter 900.
In the event that there is no wiring cross-over, the magnitude and sign data received at the input terminals 902, 904 and 906 is passed directly to the magnitude/sign converter 910, which converter converts that data into a differential data output in a manner well known in the art. In the event that a wiring cross-over is present, the sign data is inverted before being received at the magnitude/sign converter 910. Thus, the inversion that is caused by the wiring cross-over is cancelled out by an inversion introduced in the transmitter.
The transmitter 900 can be used with a receiver that does not have the selective inversion capability described above with reference to
In an exemplary implementation of the present invention, the data for transmission is quantized into one of thirteen different levels (−6, −5, −4 . . . 0 . . . 4, 5, 6) and sent at a rate of 12 GHz to a receiver. At the receiver, the signal and its inverse are applied to an ADC. As discussed elsewhere in this document, the slicing of the incoming data in such an arrangement into the correct quantum is not trivial and much correction and channel equalization may be required. A typical ADC output may be in the form of a thermometer code.
In many applications, it is desirable to convert from a thermometer code to a gray code. A receiver 1000 that carries out such a conversion is shown in
The receiver 1000 comprises an ADC 1002, thermometer code-gray code converter 1004, gray code-magnitude and sign converter 1006 and a selective inverter circuit 1008. The magnitude output of the converter 1006 and the output of the selective inverter 1008 are the outputs of the receiver 1000, which outputs are provided to the equalizer and clock recovery systems of the SerDes circuit in which the receiver can be used.
ADC 1002 receives a differential input signal from the input of the receiver 1000 and outputs a 13-bit thermometer code to thermometer code-gray code converter 1004. Thermometer code-gray code converter 1004 converts the thermometer code to a 3-bit binary magnitude signal and a sign bit. The sign bit is provided as an input to selective inverter 1008, the other input of which is a sign select input to the circuit 1000. In the event that a wiring cross-over has occurred, the sign bit will be incorrect; in that event, the sign bit received by the selective inverter 1008 is simply inverted under the control over the sign select input. If a wiring cross-over has not occurred, the sign bit will be correct and so the selective inverter 1008 does not invert the sign bit.
Thus, the arrangement of
In the present invention, it has been found that the logic required to recode the magnitude portion of the data output can introduce significant delay into the signal. This delay can be longer than the time required to recode the sign path. For this reason, dummy delay or redundant gates are typically placed in the sign path such that the data output is correctly aligned. When the sign selection of the present invention is applied, the necessary inversion may be achieved in place of one or more of the redundant gates. Thus, the present invention does not introduce any further delay in the signal processing and therefore represents a simple, elegant solution to the problem of cross-over wiring, without introducing further time delays into the data signals.
The receiver circuit of the present invention may form part of a macro in a much larger Application Specific Integrated Circuit (ASIC). In this and other situations, the sign select signal may be provided as a selection option, along with other macro configuration information. For example, such selections may be applied to a particular device by fuse selection during manufacture.
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|2||*||Beukema, "A 6.4 Gb/s CMOS SerDes Core with feed-forward and Decision Feedback Equalization" ISSCC 2005.|
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US9385893||Feb 10, 2014||Jul 5, 2016||Avago Technologies General Ip (Singapore) Pte. Ltd.||Modular low power serializer-deserializer|
|U.S. Classification||375/316, 375/259, 375/298, 375/296|
|Cooperative Classification||H04L25/08, H04L25/085|
|European Classification||H04L25/08, H04L25/08A|
|Apr 21, 2008||AS||Assignment|
Owner name: TEXAS INSTRUMENTS INCORPORATED, TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:FOREY, SIMON;REEL/FRAME:020832/0194
Effective date: 20080222
|Sep 24, 2015||FPAY||Fee payment|
Year of fee payment: 4